A typical dispatch two-way radio communication system comprises communication units, communication resources, communication sites, and a communication resource allocator. Each of the sites has a substantially distinct coverage area and is geographically located throughout the system. Each site also has a number of communication resources assigned to it, where at least one of the communication resources is used as a control channel, while a number of the remaining communication resources are used as voice channels. Such systems are known to use both frequency division multiplex access (FDMA) and time division multiple access (TDMA) methods to rebroadcast transmissions.
The communication units are typically arranged into communication groups (talkgroups) and may be located anywhere within the system (in any site). When a communication unit of a talkgroup requests a group call, it transmits, via a control channel of the site in which it is located, an inbound signaling message to the communication resource allocator. (A group call typically allows all members of the same talkgroup that are located within the system to communicate with each other.) The inbound signaling message generally comprises the requesting communication unit's individual identification number, the requesting communication unit's talkgroup, and a request for a group call. Upon receiving the inbound signaling message, the communication resource allocator will allocate a voice channel in each site to the requesting communication unit's talkgroup.
Thus, in FDMA and TDMA dispatch systems a forward link is established (one in each site where communication units are present) and monitored by all units involved in the group call in that site, and a single reverse link which is used by the group member who is currently transmitting to the other members. Non-transmitting talkgroup members are typically in a listen only mode (i.e., not able to transmit when another member is talking) and thus are not allocated a dedicated reverse link. In a TDMA system, for example, time slots are allocated to different users. A talkgroup member transmitting on an assigned reverse link, is allowed to use full power transmission to support short bursts of signals in allocated time slots. Despite the discontinuous or discrete nature of this approach, the listener receives what appears a continuous service.
In the last decade, in response to an ever-accelerating worldwide demand for mobile and personal portable communications, spread spectrum digital technology, of which one type is known as CDMA, has achieved much higher bandwidth efficiency for a given wireless spectrum allocation, and hence has proved to be an excellent alternative for serving large populations of multiple access users, than analog or other digital technologies. CDMA relies on processing power to extract a coded signal embedded across a broad frequency spectrum. The only way to extract the wanted signal from among many other overlaid unwanted signals is to have the right code. The use of coding allows more channels to be derived by the overlaying of carriers one over another and greatly enhances performance in terms of derived channels per hertz of bandwidth.
CDMA is well suited for cellular communications, but has never been employed in a conventional dispatch system. Current CDMA systems employ a form of forward power control. This means that, in a typical one-to-one (cellular) conversation, the communication unit periodically informs the base station how well it is receiving the outbound signal. If possible, the base station reduces its outbound power. If necessary, the base station increases its outbound power. This communication between the base station and the communication unit requires a two-way communication link between the two. In a dispatch situation, the most intuitive outbound power control scheme is for the base station to respond to requests to increase the power coming from any unit that requires it. However, this can only occur if all units involved in the dispatch call have an established two-way communication link.
Reverse power control is another important aspect of CDMA systems. (In fact, this is much more important than forward power control.) Reverse power control attempts to equalize the received signal powers for all communication units controlled by a particular base station. By having the base station monitoring (and accordingly vary) power from listening communication units, the efficiency of CDMA is realized.
In order to avoid excessive interference on the reverse link, communication units accessing the channel for the first time (i.e., before reverse power control has been established) use what is known as “access probes”. This means that they access the channel with low power, and slowly ramp up their power until a response from the fixed end tells them that their signal has been received. For cellular communications, the time delay caused by this access method is insignificant. On the other hand, the time delay in setting up a link for a follow-up call during a dispatch group call would be significant and must necessarily have been established in advance. In dispatch, any one of the talkgroup members may want to send an inbound message and then drop back to listening status. If each access required access probes, the delay would be intolerable for a quality dispatch service. One solution is the establishment of multiple reverse (inbound) signaling links for CDMA systems. However, these links must be established in a manner that meets the strict timing requirements of dispatch service.
In today's dispatch systems, the location of talk group members is tracked. That is, as communication units move from one base site to another, the serving base site of each unit is always known by the system. Thus, at the outset of a dispatch group call the system can determine very quickly which base sites must allocate wireless resources to support the call. CDMA cellular systems, however, use location paging. Instead of continuously tracking individual units, pages are broadcast to units during call-setup to determine which base site must support the call. Mobile units monitor a paging channel, and when paged, reply with a page response indicating their serving base site. Typically, the responding units will use access probing to minimize system interference.
While this works well for cellular calls where only one unit needs to be located and sufficient time is allowed during call-setup for the page and page response, it does not work well for locating multiple units within the tight call-setup time allowed for dispatch calls. Depending on the needs of a system's users, dispatch talkgroups can contain ten or more members and can be patched together to create even larger groups. It is likely that group members responding to a page would collide, resulting in retries and delayed responses. Moreover, the use of access probing would further delay responses. Thus, to support dispatch services comparable to those of existing dispatch systems, a CDMA system must determine which base sites are required to support a dispatch call within the strict dispatch call-setup time allowed.
Therefore, an apparatus and method for quickly establishing a CDMA dispatch call is needed.